This invention relates to radio transmission of data frames by media access based on a CSMA (carrier sense multiple access) scheme, wherein a source node forward transmits a data frame towards a destination node and retransmits the data frame or retries forward transmission of the data frame to the destination node when the source node does not receive from the destination node within an acknowledge monitor IFS (interframe space) interval following the forward transmission an ACK (acknowledgement) frame indicative of due receipt of the data frame at the destination node.
A radio data transmission system for a method of the above-described radio transmission is used typically in a radio LAN (local area network) which has a data transmission rate between 1 and 2 Mbps and is recently widely adopted to local radio data transmission. Such a radio LAN is susceptible to an influence of multipath formed by a number of radio channels which are dependent on local circumstances. The radio LAN consequently has an objectionable data transmission error rate and a deteriorated data transmission performance when compared with a wired LAN. A retransmission or retry facility is therefore specified for radio LAN's, in order to insure a sufficient data transmission performance, in a MAC (media access control) layer which is Layer 2 of the LAN reference model and corresponds to Layer 2 (data link layer) of the OSI (open system interconnection) reference model.
In the manner which will later be described more in detail, the data frame has been retransmitted as follows according to a draft standard of IEEE 802.11 Radio LAN Committee.
It will first be presumed that a data frame is correctly transmitted from a source node to a destination node so that retransmission of the data frame is unnecessary. In this case, the destination node deals with media access with an acknowledge transmission IFS interval following arrival thereat of the data frame to begin backward transmission of an ACK frame back to the source node. While put in a waiting state for forward transmission of an afresh data frame as a different frame either to the above-described source or destination node or to a different node on a transmission medium, a particular node confirms, by carrier sense or detection, disappearance of the data frame or the ACK frame from the transmission medium. A data transmission IFS interval following the disappearance of frames, the particular node begins media access for transmission of a carrier. The particular node subsequently performs a backoff process for prevention of conflict between the frames. Immediately following the backoff process, the particular node holds or secures the transmission medium and transmits the different frame to its destination.
It will now be surmised that the first-mentioned data frame is received at the destination node and includes an error. Having detected the error, the destination node does not transmit the ACK frame back to the source node. A non-response IFS interval following the forward transmission longer than the acknowledge transmission IFS interval, the source node carries out media access to transmit a carrier. Immediately following the backoff process at the source node to hold the transmission medium, the source node retransmits the first-mentioned data frame to the destination node. If this data frame is duly or correctly received, the destination node back transmits the ACK frame.
In this conventional retransmission, the source node may not receive the ACK frame from the destination node despite repetition of the retransmission because the transmission medium is subjected to a severe multipath condition. In such an event, the source node discards the data frame when the retransmission is tried a predetermined number of times.
In such a radio data transmission system, each frame includes a CRC (cyclic redundancy check) code for use in error check. Each destination node checks the CRC code. When a CRC error is detected in the data frame, the destination node does not produce the ACK frame. When the data frame is discarded in the MAC layer or by a MAC protocol, retransmission is assigned to a higher layer and is processed by, for example, a TCP (transmission control protocol) of the TCP/IP (transmission control protocol/internet protocol). Such a retransmission facility gives a high reliability to data transmission in the MAC layer.
In the manner described in the foregoing, three different IFS intervals are used for the carrier sense in the retransmission facility in order to give a higher priority degree at a general destination node to the backward transmission of the ACK frame. More particularly, vacancy or idleness of the transmission medium is detected at the destination node in general the acknowledge transmission IFS interval following arrival thereat of a data frame with the acknowledge transmission IFS interval predetermined shorter than the data transmission IFS interval and with the backoff process omitted. In other words, the destination node backward transmits the ACK frame, with no restriction, immediately after a lapse of the acknowledge transmission IFS interval. The non-response IFS interval is naturally longer than the acknowledge transmission IFS interval and is shorter than the data transmission IFS interval.
The backoff process is performed at the source station following the non-response IFS interval. The backoff process is performed also at each node that may be whichever of the source and the destination nodes concerned previously with a data frame and an ACK frame and of a different node called the particular node before. The transmission medium is actually accessed by one of such waiting nodes waiting for access to the transmission medium that is settled by a least random number at the lapse of the data transmission IFS interval.
In the meanwhile, an improved retransmission or retry technique is disclosed in Japanese Patent Prepublication (A) No. 154,238 of 1992. According to this patent prepublication, a plurality of work stations are connected as nodes to a LAN, which is connected in turn to a server for the work stations. Each work station comprises first means for requesting retransmission with a presumption of disappearance of a data frame upon failure of an acknowledgement frame within a predetermined retry time interval and second means for adjusting the retry time interval on a basis of an overloaded state of the server. This insures a high reliability of data transmission and reception with no deterioration of system performance even if it is impossible to neglect the retry time interval in an overloaded state, namely, when the retry time interval would further increase the overload state of the server.
Most recently, it has become indispensable for a radio LAN to deal with multimedia information which includes image data, picture information, and others besides character d ata. As a consequence, the data transmission rate must be as high as 10 Mbps or higher. This necessitates a higher symbol rate and makes it necessary to strengthen a withstandingness against the multipath. This further necessitates more frequent occurrence of the retransmission because a frame error rate rises under common multipath circumstances (a common delay time of a reflected signal for a main signal).
It is therefore summarized in connection with a conventional radio data transmission system that the system has a deteriorated throughput as a whole. This is because the retransmission is more frequently necessary in a recent radio LAN of a higher data transmission rate due to severe multipath circumstances and because the retransmission is assigned to a higher layer when the retransmission in a lower layer, such as by the MAC protocol, is frequently necessary as often as a predetermined limit of frequency.
Even by the improved retransmission technique, a deterioration is inevitable in the throughput or in an overall performance of the system because the retransmission becomes inevitably more frequent under severe circumstances of the transmission medium.